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Swine Influenza—or "swine flu"—is a respiratory disease of pigs caused by type A influenza virus. Regular outbreaks of swine flu in pigs occur each year. Swine flu outbreaks infect large numbers of pigs but the death rate is low. Swine flu viruses circulate among the world's pig population throughout the year, but most outbreaks occur during the late fall and winter months. The swine flu virus (an influenza type A H1N1 virus) was first isolated from a pig in1930.

There are presently four main influenza type A virus subtypes affecting pigs: H1N1, H1N2, H3N2, and H3N1. Most of the recently isolated flu viruses from pigs have been H1N1viruses.

Influenza viruses can be directly transmitted from pigs to people and from people to pigs. Humans do NOT get swine flu from eating pork. Pigs frequently contract human flu. They are also susceptible to avian flu. Swine flu can spread to humans who are in close  proximity to infected pigs (and vice versa), such as in pig barns and livestock exhibits housing pigs at fairs. Human-to-human transmission of swine flu has been infrequent. When it occurs it is transmitted in the same way seasonal flu spreads in people.  Person-to-person transmission of the flu usually happens through coughing or sneezing of people infected with the influenza virus. People may become infected by touching something with flu viruses on it and then touching their mouth or nose. People are contagious from about a day before they have symptoms to seven days after the onset of a flu.

In March and April 2009, more than 1,000 cases of swine flu in humans were detected in Mexico,and more than 80 deaths are suspected to have a connection with the virus. As of April 26, 2009 there were 20 laboratory confirmed cases in the United States and a public health emergency was declared. The Center for Disease Control is carefully investigating the outbreak in humans and documenting changes at this link.